The combustion temperatures in current gas turbine engines are high enough to melt the alloys used in the “hot path” components, and these temperatures continue to rise as gas turbine engines are further developed. As a consequence, many of the components must be cooled using a gaseous cooling medium passed through complex channels within the “hot path” components. To further protect the hot path components, a thermally insulating protective layer can be used. The temperature gradient over the Thermal Barrier Coating (TBC) is high, reducing the temperature to which the alloys of the hot path components are exposed.
Conventionally, cooling channels have been placed inside the alloy, relatively far from the “hot surface.” Improvements have moved the cooling channels closer to the hot surface, and some channels are formed at the interface of the alloy and the thermal barrier coating. This approach leaves more of the alloy on the cool side of the channels than in earlier designs, which ultimately results in increased longevity of the turbine components.
Casting complex cooling channels can be extremely complex and expensive, particularly in large components. Consequently, various methods of manufacturing cooling channels near the surface have been explored and are known in the art. Powder salt has been used to fill grooves on the surface of a substrate casting prior to plasma spraying of metal onto the substrate surface and over the salt filled grooves. However, the form and surface of the channel can be difficult to control using this technique, and salt has been known to migrate out of the groove during the manufacturing process.
Another method, described in U.S. Pat. No. 6,921,014, issued to Hasz et al., includes applying a “stop-off” material to the metal substrate, and then applying a bonding agent. The stop-off material prevents adhesion between the bonding agent and the substrate. Additional layers are subsequently applied to the bonding agent. What results is a substrate bonded to a bonding agent except where the stop-off material was applied, where there remains a gap. This gap can serve as a cooling channel, and the stop-off material may remain or may be removed. As shown by FIG. 3 of U.S. Pat. No. 6,921,014, the form of the resulting channel can be difficult to control, resulting in stress risers.
Yet another method described in U.S. Pat. No. 6,921,014 includes applying a layer to a substrate, where the layer contains a bonding agent together with a “sacrificial material,” where various materials can be used as sacrificial materials. Additional layers are then applied. Subsequently, the sacrificial material is removed, leaving a cooling channel between the substrate and the additional layers. Other methods, such as described in U.S. Pat. No. 6,321,449 issued to Zhao et al., include filling grooves in the substrate with pastes or slurry, applying additional layers, and then removing the filler material. These methods typically result in a cooling channel with a four sided cross section, and sharp, 90 degree corners in the cross section.